Fixation and fate of C and N in the cyanobacterium Trichodesmium using nanometer-scale secondary ion mass spectrometry

The marine cyanobacterium Trichodesmium is ubiquitous in tropical and subtropical seas and is an important contributor to global N and C cycling. We sought to characterize metabolic uptake patterns in individual Trichodesmium IMS-101 cells by quantitatively imaging ¹³C and ¹⁵N uptake with high-resolution secondary ion mass spectrometry (NanoSIMS). Trichodesmium fix both CO₂ and N₂ concurrently during the day and are, thus, faced with a balancing act: the O₂ evolved during photosynthesis inhibits nitrogenase, the key enzyme in N₂ fixation. After performing correlated transmission electron microscopy (TEM) and NanoSIMS analysis on trichome thin-sections, we observed transient inclusion of ₁₅N and ₁₃C into discrete subcellular bodies identified as cyanophycin granules. We speculate that Trichodesmium uses these dynamic storage bodies to uncouple CO₂ and N₂ fixation from overall growth dynamics. We also directly quantified both CO₂ and N₂ fixation at the single cell level using NanoSIMS imaging of whole cells in multiple trichomes. Our results indicate maximal CO₂ fixation rates in the morning, compared with maximal N₂ fixation rates in the afternoon, bolstering the argument that segregation of CO ₂ and N₂ fixation in Trichodesmium is regulated in part by temporal factors. Spatial separation of N₂ and CO₂ fixation may also have a role in metabolic segregation in Trichodesmium. Our approach in combining stable isotope labeling with NanoSIMS and TEM imaging can be extended to other physiologically relevant elements and processes in other important microbial systems.